The present invention relates to a method and to an apparatus for producing nuclear reactor structural elements, such as spacing grids by pulse laser welding and more generally to the application of such a method to the production of devices incorporating small thin parts which are to be welded together.
In order to prevent lateral vibrations of fuel rods in nuclear reactors, spacing grids or mixing grids are arranged every so often along the assemblies. These grids are constituted by a system of thin plates, which intersect in pairs and thus define a certain number of racks in which are held the rods. The cooling liquid circulates between these plates and the rods and it is possible to equip the grids with so-called mixing fins, whose plane forms an angle with that of the thin plates in order to deflect the liquid streams and homogenize the cooling liquid. The mixing grids are generally very small (plate thickness approximately a few tenths of a millimeter, whilst the rack dimensions are approximately 10 to 20 mm).
At present, mixing grids are usually made from Inconel, an alloy which is easy to assemble to brazing or hard soldering, but this metal has the disadvantage of being a good neutron absorber, which causes long life activation products, such as Cobalt 60, in the primary circuit. For this reason, attention has been directed towards other alloys and in particular zirconium alloys, such as Zircaloy 4. The latter have a good resistance to corrosion by water, but are much more difficult to weld than Inconel and for this reason they have not hitherto been used for this purpose.
At first, Zircaloy is difficult to braze and this method cannot be envisaged. Moreover, as the grids are small, positioning problems are encountered on using torches and there is a danger of serious heating of the metal, which can cause deformations and oxidation of the plates.